There are below-discussed known light emitting element control devices.
As shown in FIG. 33, a light emitting element control device disclosed by 1 Japanese Unexamined Patent Application No. 167139/1989 (Tokukaihei 1-167139) includes an amplifier 50, a light emitting element 51, a light receiving element 52, an A/D converter 53, a CPU 54, and a D/A converter 55. The light emitting element 51 and the light receiving element 52 constitute an optical sensor, and by a control voltage signal to be output through the D/A converter 55 from the CPU 54, an amount of light emitted from the light emitting element 51 is adjusted. On the other hand, the light receiving element 52 detects an amount of light emitted from the light emitting element 51, and after the amount of light is amplified by the amplifier 50, it is inputted to the CPU 54 through the A/D converter 53. The CPU 54 adjusts a control voltage signal value of the D/A converter 55 so that the amount of light falls in a range of from a predetermined upper limit value to a predetermined lower limit value. As a result, the described control device controls the light emitting element.
As shown in FIG. 34, the light emitting element control device disclosed by 2 Japanese Unexamined Patent Application No. 271025/1992 (Tokukaihei 4-271025) includes a CPU 56, a laser diode 57, a D/A converter 58, a driving circuit 59 for a laser diode 57, a pin monitor 60, an A/D converter 61 and a variable circuit 62.
The laser diode 57 is arranged so as to emit light in accordance with a light amount indicative value. The light amount indicative value is output from the CPU 56, and is sent to a driving circuit 59 through the D/A converter 58. On the other hand, an amount of light emitted from the laser diode 57 is detected by the pin monitor 60, and a detected amount of light is inputted to the CPU 56 via the A/D converter 61. The CPU 56 calculates a difference between the light amount indicative value and a currently detected amount of light, and the light amount indicative value is adjusted by an APC (auto/power/control) circuit. Here, in order to maintain the control precision of the APC high, an operational amplifier 62a included in the variable circuit 62 varies a gain of the detection signal detected by the pin monitor 60, and a value of a signal output from the pin monitor 60 varies in response to a change in gain to be output to the A/D converter 61. As a result, the described control device controls the light emitting element.
3 Japanese Unexamined Patent Application No. 1674/1992 (Tokukaihei 4-1674) discloses a light emitting element control device designed for a blank lamp control device provided in a copying machine. The blank lamp is provided for removing charges from a non-image-forming area on a drum-shaped photoreceptor in the case of carrying out a copying operation in a reduced size or in a frame elimination mode. For example, in a single sided copying machine, as shown in FIG. 35, the blank lamp 63 is provided facing the photoreceptor drum 68 so as to remove only charges of a so-called out of maximum image area (slashed line), i.e., an area obtained by subtracting the maximum image area B to which an image is copied from the drum width A of the photoreceptor drum 68.
The above-mentioned reference 3 discloses a control device for a blank lamp 63, wherein the PWM signal (pulse width modulation signal) output from control means 64 is used as a control voltage signal for the blank lamp 63 in an integrating circuit 65, and 10 blank lamps 63 (63a.about.63j) are controlled to be lighted up by the control voltage signal as shown in FIG. 36.
Specifically, as shown in FIG. 37, by setting the pulse width of the PWM signal set in a predetermined interval into 10 levels, a lighting control of 10 lamps is permitted. To be more specific, the PWM signals are set by inputting 10 pulse widths in the register section in the control means 64, and are output from the PWM signal generation port in the control means 64.
The PWM signal transmitted from the control means 64 is sent to the integrating circuit 65 shown in FIG. 36, and by passing through the integrating circuit 65, the PWM signal is converted into the voltage signal which linearly varies in response to the pulse width. Then, the voltage signal is inputted to the lamp driving circuit section 66, and the blank lamps 63 in the same number as the inputted voltage signals light up.
The lamp driving circuit section 66 includes 10 circuits. Each circuit is arranged such that the comparative amplifiers 67 (67a-67j), and the blank lamps 63 (63a-63j) composed of LED (Light Emitting Diode), etc., are connected in series respectively. To a positive terminal of each comparative amplifier 67, a control voltage signal converted by the integrating circuit 65 is applied, and to a negative terminal of each comparative amplifier 67, a reference voltage v is applied. The reference voltage v is calculated based on a relationship between a pulse width of the PWM signal (converted into a duty ratio) and an output voltage V.sub.0 of the control voltage signal. Then, the power source voltage V.sub.cc divided by the resistor in accordance with a level of a required output voltage V.sub.0 (from V.sub.1 to V.sub.10) for lighting on 10 blank lamps 63.
In the case of carrying out an equivalent size copying, as the PWM signal is not generated, any of the blank lamps 63 does not light on. However, for example, in the case of outputting the PWM signal of level 1 having a minimum pulse width shown in FIG. 37, a control voltage signal, i.e., an output voltage V.sub.1 shown in FIG. 38 is inputted to the lamp driving circuit section 66, and only the blank lamp 63a lights on. Thereafter, according to a pulse width level of the PWM signal, blank lamps 63b through 63j light on accordingly. For example, when carrying out a copying operation with a reduced copy size from B-4 size to B-5 size, the magnification is 70 percent (50 percent in area), and the pulse width of the PWM signal corresponds to the maximum level 10. Therefore, the control voltage signal of the output voltage V.sub.10 is inputted to the lamp driving circuit section 66, and all of the 10 blank lamps (63a through 63j) light on.
However, both the optical sensor control device of the reference 1 or the APC circuit of the reference 2 require an A/D converter in a path for feeding back a detected output of the light receiving element and the pin monitor to the CPU, and the operation voltage of the light receiving element and the pin monitor are set to the power source voltage (5V) of the CPU, or as the amplifier 50 or the operation amplifier 62a shown in FIG. 33 and FIG. 34, an amplifying/attenuating circuit, etc., is provided on the light receiving side, and the output voltage is required to be set equivalent to the power source voltage.
On the other hand, in the control device of the blank lamp in the reference 3, the number of blank lamps indicates a number of groups of blank lamps classified in such a manner that lamps in each group light up simultaneously and lamps in different groups light up at different timing. As the number of groups increases, a lighting control cannot be performed stably without adopting resistors of high precision in the previous circuit to the control voltage signal generating circuit.
The described conventional arrangement is designed for controlling the lighting of the blank lamps of at most 10 groups, and thus only 10 kinds of reference voltages v.sub.1 through v.sub.10 are required for the comparative amplifier 67 of the lamp driving circuit section 66 shown in FIG. 36, and a difference in voltage between the reference voltages (between v.sub.1 and v.sub.2, and between v.sub.2 and v.sub.3 . . . ) is around 1.6 (V) as shown in Table 39. Here, the power source voltage V.sub.cc before being divided by the resistor is 18 (V).
However, when the described conventional arrangement is applied for controlling the lighting of more than 10 groups, for example, 15 groups, and for controlling lighting off of all the lamps in the 15 groups, 16 kinds of the reference voltages v.sub.1 through v.sub.16 are required for the comparative amplifier 67, and a difference between the reference voltages (between v.sub.1 and v.sub.2, and between v.sub.2 and v.sub.3, . . . ) becomes smaller (around 1.1 (V)).
As the difference in voltage between the reference voltages becomes smaller, in order to stably control a lighting of the blank lamp 63 with a comparative output from the comparative amplifier 67, an error of the output voltage V.sub.0 is required to be set small. This requires a high precision resistor, etc., which causes an increase in cost.